The Biology of Lichens

An introduction to the fascinating world of lichens.

A closer look at a tree trunk or a stone surface can reveal a world of crusts, leafy lobes and miniature branches. Lichens invite us to explore how organisms live together, respond to their surroundings and contribute to biodiversity.

A lichen develops through an association between a fungus and a photosynthetic partner—a green alga, a cyanobacterium, or sometimes both. The fungus forms most of the visible structure, while the photosynthetic partner supplies carbon compounds produced using light.

Lichens are classified according to their fungal partner. Although some look like tiny plants, they belong within the fungal kingdom (Honegger, 1991; Nash, 2008).

Observe and Explore: Find a lichen patch and consider how its partners might obtain light, water and a place to live.

The fungus is called the mycobiont, and its photosynthetic partner the photobiont. Together, they develop a body and way of life that differ from those of the partners growing separately (Honegger, 1991).

Observe and Explore: Consider how this partnership can persist on an exposed surface.

The lichen body is called a thallus. In many species, photosynthetic cells lie beneath a protective surface layer, or cortex, with more loosely arranged fungal threads forming the medulla below.

Internal organisation varies among lichens. In many foliose species, structures called rhizines anchor the lower surface to its substrate. They are attachment structures rather than the nutrient-absorbing roots of plants.

The thallus must allow light and carbon dioxide to reach the photobiont while responding to changing moisture conditions (Büdel & Scheidegger, 2008).

Observe and Explore: Examine a naturally exposed lobe edge. Can you distinguish the upper surface from the underside?

Growth form provides a useful first step when examining a lichen.

Crustose lichens form crusts closely attached to, or partly immersed in, their substrate. Foliose lichens have flattened lobes with distinguishable upper and lower surfaces. Fruticose lichens produce branching structures that may stand upright or hang down.

Some lichens instead form small scales, powdery patches or combinations of structures. These categories describe appearance rather than a complete evolutionary classification (Büdel & Scheidegger, 2008).

Observe and Explore: Look for contrasting growth forms. Observe their attachment without attempting to lift or remove them.

Fruticose and filamentous forms

At the Royal Botanic Gardens, Kew, fruticose and filamentous lichens represent two visually distinct growth forms. Fruticose lichens are three-dimensional, shrubby or hair-like, and often branched, growing upright or hanging from twigs and branches. Common examples include Usnea, Ramalina, and Evernia, which thrive in well-lit, well-ventilated habitats and respond sensitively to air quality.

In contrast, filamentous lichens consist of fine, thread-like strands closely associated with moist, shaded surfaces, sometimes resembling algae. These forms lack the robust branching of fruticose lichens and are typically found in humid microhabitats. Distinguishing these growth forms helps observers interpret habitat conditions and lichen ecology across Kew.

Lichen activity changes with water availability. Most lichens can tolerate substantial drying, during which photosynthesis and other metabolic processes become greatly reduced. Rehydration allows activity to resume when conditions are suitable.

This ability is called desiccation tolerance. It involves several protective mechanisms; tolerance and recovery differ among species and environmental conditions (Kranner et al., 2008).

Light is also essential, but more light is not always better. The partnership must balance access to sunlight with protection from excessive exposure. Wetting can change a lichen’s appearance as its cortex becomes more transparent (Büdel & Scheidegger, 2008; British Lichen Society, n.d.-a).

Observe and Explore: Photograph the same patch after rain and during dry weather. Compare its colour and appearance.

Lichens can disperse with their partners already together. Soredia are small, powdery units containing fungal threads and photobiont cells. Isidia are corticated outgrowths that can detach. Fragments of the thallus provide another route for dispersal.

The fungal partner can also reproduce sexually. In many species, spores develop in structures such as disc-shaped apothecia or flask-shaped perithecia. A spore does not carry the complete partnership: the developing fungus must encounter a compatible photobiont to form a new lichen.

Some species use both sexual reproduction and vegetative dispersal (Honegger & Scherrer, 2008; British Lichen Society, n.d.-c).

Observe and Explore: Look for powdery areas, tiny projections and fruiting bodies. These structures offer clues to reproduction and identification.

Lichen colour reflects pigments, anatomy and hydration. Chemical compounds can also influence how lichens interact with light and grazing animals.

Some substances in the cortex absorb or reflect radiation, helping protect the photobiont from excessive light. Parietin, the orange pigment associated with Xanthoria parietina, is one example of a light-screening compound.

Other secondary compounds can discourage some herbivores. These functions vary: not every compound acts as a sunscreen, and not every lichen relies on the same chemical defences (Solhaug & Gauslaa, 2012).

Observe and Explore: Record the colour of the thallus separately from that of its fruiting bodies. Use colour as a clue rather than a complete identification.

Using Colour for Quick Identification

Colour is a practical first clue for quickly recognising lichens at the Royal Botanic Gardens, Kew. Bright yellow and orange lichens often indicate sun-exposed, nitrogen-enriched habitats, while pale greys and whites are common on clean bark and stone. Greenish tones typically appear when lichens are wet and actively photosynthesising, fading as they dry. Dark browns and blacks may signal species adapted to high light or exposed surfaces. Although colour alone is not sufficient for precise identification, it provides an effective starting point in the field. Combined with growth form, substrate, and habitat, colour helps visitors and researchers rapidly narrow down likely lichen groups at Kew.

Texture, Shape, and Surface Features

Lichens at the Royal Botanic Gardens, Kew display a remarkable variety of textures, shapes, and surface features that aid identification. Smooth, cracked, powdery, or warty surfaces reflect different growth strategies and environmental adaptations. Crustose lichens form thin, tightly attached layers on bark and stone, while foliose species develop leaf-like lobes with distinct upper and lower surfaces. Fruticose lichens grow as branched or hair-like structures, often hanging from twigs. Surface features such as soredia, isidia, apothecia, and fissures provide further diagnostic clues. Together, texture and form reveal how Kew’s lichens respond to light, moisture, substrate, and air quality across the Garden.

Interpreting Lichen Anatomy in the Field

Interpreting lichen anatomy in the field relies on careful observation rather than microscopes. Features such as growth form—crustose, foliose, or fruticose—provide the first anatomical clues. The presence of an upper and lower cortex, rhizines, lobes, or branching patterns helps distinguish major groups. Surface structures like soredia, isidia, apothecia, and fissures can often be seen with a hand lens. Colour changes when wet or dry also reflect internal anatomy and water storage. By combining these visible traits with substrate and habitat, field observers can interpret lichen anatomy effectively and make confident preliminary identifications before laboratory study.

Differentiating Algae and Lichens

Although algae and lichens may look similar in the field, they are fundamentally different organisms. Algae are single-celled or simple multicellular photosynthetic organisms that grow directly on damp surfaces, often forming green films or slimy layers. Lichens, in contrast, are stable symbiotic associations between a fungus and a photosynthetic partner (an alga or cyanobacterium). This partnership produces a structured thallus with distinct forms—crustose, foliose, or fruticose—that is usually dry, firmly attached, and slow growing. Lichens lack the slimy texture typical of algae and persist through drought by entering dormancy. Observing texture, structure, attachment, and response to drying helps reliably distinguish lichens from algae in natural and garden settings.

Common Look-Alikes and How to Avoid Confusion

Many lichens appear deceptively similar, especially to beginners. Grey foliose species such as Parmelia and Hypogymnia are often confused; checking the underside for rhizines and whether the lobes are hollow can quickly separate them. Yellow-orange lichens may belong to Xanthoria (foliose) or Caloplaca (crustose), so noting growth form is essential. Dark crustose lichens on stone are frequently mistaken for algae—true lichens feel dry and firmly attached. Using a hand lens to examine texture, reproductive structures, and attachment, and always considering substrate and habitat, greatly reduces confusion and leads to more confident lichen identification.

Identifying Crustose Lichens on Stone

Crustose lichens on stone are among the most common yet challenging lichens to identify at the Royal Botanic Gardens, Kew. These species form thin, tightly attached crusts that cannot be removed without damaging the substrate. Colour is a useful first clue: pale grey and white crusts often belong to genera such as Lecanora or Lecidella, while yellow and orange patches suggest Caloplaca or Xanthoria. Black or dark crusts on damp stone are frequently Verrucaria. Surface texture, cracking patterns, and the presence of apothecia or perithecia provide further guidance. Combined with substrate type and moisture conditions, these features allow reliable preliminary identification in the field.

Rapid identification of common lichens in the Royal Botanic Gardens, Kew can begin by grouping species according to their substrate. Stone lichens are often crustose, tightly attached, and pale grey, white, yellow, or black, commonly belonging to genera such as Lecanora, Caloplaca, and Verrucaria. Twig and branch lichens are frequently fruticose or small foliose forms, including Ramalina, Usnea, and Evernia, adapted to light and air movement. Tree trunk lichens show greater diversity, with foliose and crustose genera such as Parmelia, Physcia, Graphis, and Lecidella. On historic surfaces, slow-growing crustose species dominate, reflecting substrate chemistry and long-term environmental conditions. This substrate-based approach provides a practical rapid key for garden exploration.

Microscopic Characters of Lichens

Microscopic characters are essential for accurate lichen identification and reveal features invisible to the naked eye. Examination of thin sections under a microscope shows the internal structure of the thallus, including the cortex, photobiont layer, and medulla. Reproductive structures provide key diagnostic traits: the shape, size, and septation of ascospores; the form of asci and paraphyses; and the anatomy of apothecia or perithecia. Pycnidia and conidia, when present, offer further clues. These microscopic features, combined with external morphology and chemistry, allow reliable species identification and help clarify relationships among closely related lichen taxa.

Spot Tests and Chemical Indicators

Spot tests are simple chemical reactions used to help identify lichen species by revealing diagnostic compounds. Small drops of reagents—commonly potassium hydroxide (K), sodium hypochlorite (C), and para-phenylenediamine (P)—are applied to the thallus or medulla. Colour changes such as yellow, red, orange, or purple can indicate specific secondary metabolites. Under UV light, some lichens fluoresce brightly, providing additional clues. While spot tests do not replace microscopic or molecular analysis, they are valuable field and laboratory tools. Combined with morphology, substrate, and habitat, chemical indicators greatly improve the accuracy of lichen identification.

What is a lichen species?

The concept of a lichen species can no longer be restricted to the classical view of a mutualistic association between a single fungal partner and one photosynthetic partner. Traditionally, species delimitation in lichens relied primarily on the mycobiont, as the fungal component governs morphology, development, and secondary chemistry. However, recent research has demonstrated that the lichen thallus is a complex multi-organism symbiotic system, hosting multiple algal lineages, yeasts, bacteria, and even viruses Morillas et al. (2022).

Within this modern framework, a lichen species represents a multi-species symbiotic unit, whose structure and function emerge from dynamic interactions among the mycobiont, one or more photobionts, and the lichen microbiome. This microbiome—comprising diverse microorganisms inhabiting the surface or internal layers of the thallus—contributes directly to ecological performance, stress tolerance, metabolic capabilities, and the production of bioactive compounds.

Thus, a lichen species may be understood as a fungus-defined symbiotic system, whose identity and ecological behavior are fundamentally shaped by its associated microbial consortium. Consequently, species boundaries in lichens should be delimited not solely by fungal morphology or genetics, but also by considering the composition, stability, and functional integration of the full symbiotic community that collectively forms a persistent and coherent thallus.

Ref:

Morillas, L., Roales, J., Cruz, C., & Munzi, S. (2022). Lichen as multipartner symbiotic relationships. Encyclopedia, 2(3), 1421-1431.

DNA barcoding and modern taxonomy

At the Royal Botanic Gardens, Kew, DNA barcoding has become central to modern lichen taxonomy. By sequencing standard markers such as ITS, mtSSU, and RPB genes, Kew researchers resolve cryptic species, clarify synonymy, and stabilise names that morphology alone cannot. Integrated with the world-leading Fungarium collections and historical type material, molecular data link past records to present diversity. This approach strengthens biodiversity inventories, supports conservation assessments, and underpins accurate identification for ecology, bioindication, and heritage science within this UNESCO World Heritage Site.

At the Royal Botanic Gardens, Kew, lichen photography opens a window onto hidden micro-worlds often overlooked by the naked eye. Using macro lenses and simple hand lenses, photographers reveal intricate colours, textures, and growth forms that define lichen diversity. These images support identification, documentation, and public engagement, transforming crusts on bark, stone, and walls into striking visual stories. At Kew, lichen photography bridges science and art, helping visitors appreciate the beauty, complexity, and ecological significance of these remarkable symbiotic organisms within a UNESCO World Heritage Site.

At the Royal Botanic Gardens, Kew, lichens are interpreted not only as biological organisms but also as part of human cultural history. Across Europe and beyond, lichens have featured in traditional dyes, folk medicine, perfumery (such as oakmoss), and rural knowledge systems. Kew’s collections and archives preserve historical references that link lichen species to textile colouring, wound treatments, and symbolic meanings in folklore. By presenting these stories alongside modern science, Kew highlights how lichens connect ecology, culture, and traditional knowledge, enriching our understanding of their value beyond taxonomy and conservation.

British Lichen Society (n.d.-a). Lichen Biology. Online resource.

British Lichen Society (n.d.-c). Lichen Life Cycle. Online resource.

British Lichen Society (n.d.-d). Studying Lichens. Online resource.

Büdel, B. & Scheidegger, C. (2008). Thallus morphology and anatomy. In: Nash, T.H. III (ed.), Lichen Biology. 2nd edition. Cambridge University Press, pp. 40–68. DOI.

Dobson, F.S. (2018). Lichens: An Illustrated Guide to the British and Irish Species. 7th edition, reprint. Richmond Publishing. Book information.

Honegger, R. (1991). Functional aspects of the lichen symbiosis. Annual Review of Plant Physiology and Plant Molecular Biology, 42, 553–578. DOI.

Honegger, R. & Scherrer, S. (2008). Sexual reproduction in lichen-forming ascomycetes. In: Nash, T.H. III (ed.), Lichen Biology. 2nd edition. Cambridge University Press, pp. 94–103. Chapter information.

Kranner, I., Beckett, R., Hochman, A. & Nash, T.H. III (2008). Desiccation-tolerance in lichens: A review. The Bryologist, 111(4), 576–593. DOI.

Nash, T.H. III (ed.) (2008). Lichen Biology. 2nd edition. Cambridge University Press. DOI.

Smith, C.W., Aptroot, A., Coppins, B.J., Fletcher, A., Gilbert, O.L., James, P.W. & Wolseley, P.A. (eds) (2009). The Lichens of Great Britain and Ireland. British Lichen Society. Book and revision information.

Solhaug, K.A. & Gauslaa, Y. (2012). Secondary lichen compounds as protection against excess solar radiation and herbivores. In: Lüttge, U., Beyschlag, W., Büdel, B. & Francis, D. (eds), Progress in Botany 73. Springer, pp. 283–304. DOI.